Hybrid poplar leaf characteristics related to drought resistance

Austin J. Himes and Heidi J. Renninger, Department of Forestry, Mississippi State University, Starkville, MS, Brian J. Stanton, GreenWood Resources Inc., Portland, OR, Todd N. Rosenstiel, Biology, Portland State University, Portland, OR, Rose McClung, Portland State University, Paul Emerson, GreenWood Resources Inc.

Background/Question/Methods
Populus spp. and their hybrids have been used as production crops for bioenergy and as model system for studying aspects of forest ecology, tree physiology and woody plant genetics. Both natural forests and production ecosystems are likely to face increased drought through much of the temperate zone as a result of climate change. Identifying easy, inexpensive indicators of what species or varieties of trees are likely to be resistant to drought (maintain higher levels of net primary productivity with reduction in plant available water) could be an important step for adapting managed systems to future climates and predicting climate change impacts on natural systems. We conducted a field study manipulating plant water availability to hybrid poplar varieties from different taxa with contrasting water use strategies, as indicated by leaf tissue Δ13C. Our goal was to identify leaf traits that could reliably predict drought resistance. In addition to Δ13C we measured leaf mass per area (LMA), petiole diameter (PD), petiole length (PL), individual leaf area (LA), adaxial stomatal density (ADAX), abaxial stomatal density (ABAX), minor vein density (MVD) and chlorophyll content (CHLOR). We observed above ground biomass accumulation of the different varieties under reduced water availability relative to well water conditions as an indicator of drought resistance and used structured equation modeling to identify which leaf characteristics were most related drought resistance.
Results/Conclusions
We found that ABAX was the only single trait that was a statistically significant predictor of varietal performance under drought stress. However, the best indicator was a composite of all nine leaf traits considered. ABAX as well as composite indicators based on structural equation modeling should be useful tools for identifying drought resistant varieties of hybrid poplar that could be deployed for production or service plantations on water limited sites.